Stimulation procedures often require the use of well treating materials having high compressive strength. In hydraulic fracturing, such materials must further be capable of enhancing the production of fluids and natural gas from low permeability formations. In a typical hydraulic fracturing treatment, fracturing treatment fluid containing a solid proppant material is injected into the wellbore at high pressures. Once natural reservoir pressures are exceeded, the fluid induces fractures in the formation and proppant is deposited in the fracture, where it remains after the treatment is completed. The proppant material serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the wellbore through the fracture. Because fractured well productivity depends on the ability of a fracture to conduct fluids from a formation to a wellbore, fracture conductivity is an important parameter in determining the degree of success of a hydraulic fracturing treatment. Choosing a proppant is critical to the success of well stimulation.
Proppants used in the art include sand, glass beads, walnut hulls, and metal shot as well as resin-coated sands, intermediate strength ceramics, and sintered bauxite; each employed for their ability to cost effectively withstand the respective reservoir closure stress environment. The relative strength of these various materials increases with their corresponding apparent specific gravity (ASG), typically ranging from 2.65 for sands to 3.4 for sintered bauxite. Unfortunately, increasing ASG leads directly to increasing degree of difficulty with proppant transport and reduced propped fracture volume, thereby reducing fracture conductivity.
More recently, ultra lightweight (ULW) materials have been used as proppants since they reduce the fluid velocity required to maintain proppant transport within the fracture, which, in turn, provides for a greater amount of the created fracture area to be propped. Exemplary of such proppants are significantly lighter deformable particles. Such ULW proppants, like conventional heavier proppants, have the capability to effectively withstand reservoir closure stress environments while increasing fracture conductivity.
Materials of various specific gravities have been disclosed for use as deformable particulates for downhole conditions. For example, successful deformable particles include modified ground walnut hulls manufactured by impregnating closely sized walnut particles (i.e. 20/30 US mesh) with epoxy or other resins. These impregnated walnut hull particles are then coated with phenolic or other resins. Further exemplary of deformable particles are polystyrene divinylbenzene (PSDVB) deformable beads.
In addition to having low specific gravity, ULW proppants must also be of sufficient strength to withstand the rigors of high temperatures and high stresses downhole. ULW proppants, while offering excellent compressive strength, often soften and loose their compressive strength especially at high temperature and high pressure conditions. For instance, ULW proppants composed of resinous materials have been observed to deform at elevated temperatures to the extent that under a 5,000 psi stress load at temperatures greater than 250° F., the permeability of the ULW proppant pack is deformed beyond the limits of its commercial utility even though the melting point of the resin is at a temperature of well greater than 300° F.
Thus, alternative materials which exhibit high particle strength at high temperatures are needed for utilization in those applications which require high temperature and high pressure downhole conditions.